An IWAD file from the original DOOM, Heretic or Hexen is
required. IWADs use the filename extension .WAD and can be
found in the directory where you have the original game installed.
They are typically over 10 MBs in size.
Updating the original game to the latest available version will
save you from many problems. A patch for Hexen is located here at
DHQ and patches for the other games
can quite easily be found using e.g. Google.
See this F.A.Q. entry for
more links.
